An electromagnetic wave having a frequency around 1 THz, that is, a far infrared ray and a submillimeter wave in this region is referred to as a “terahertz wave”. The terahertz wave is positioned at a boundary between a light wave and a radio wave, and has characteristics of both of the light wave and the radio wave.
One of characteristics of the terahertz wave is that the wave is the shortest wavelength band having a substance transmitting property of the radio wave and the longest wavelength including a rectilinear property of the light wave. That is, the terahertz wave can be transmitted through various substances as in the radio wave, and has a short wavelength (around 1 mm to around 30 μm). Therefore, the highest space resolution is obtained in a radio wave band. Moreover, like the light wave, the terahertz wave can be drawn by a lens and a mirror.
FIG. 1A is a generation principle diagram of this terahertz wave. In this drawing, reference numeral 1 is a non-linear optical crystal (e.g., LiNbO3), 2 is pump light (or referred to as excitation light), 3 is idler light, and 4 is the terahertz wave. It is to be noted that the pump light 2 and the idler light 3 are infrared light having a wavelength of around 1 μm.
When the pump light 2 is incident into the non-linear optical crystal 1 having the Raman activity and a far infrared activity in a constant direction, the idler light 3 and the terahertz wave 4 are generated via a raw excitation wave (a polariton) of the substance by stimulated Raman scattering (or a parametric interaction). In this case, among the pump light 2 (ωp), the terahertz wave 4 (ωT) and the idler light 3 (ωi), a law of conservation of energy represented by Equation (1) and a law of conservation of momentum (a requirement for phase matching) represented by Equation (2) are established. It is to be noted that Equation (2) is a vector, and a requirement for non-colinear phase matching can be satisfied as shown in FIG. 1B.ωp=ωT+ωi  (1); andκp=κT+κi  (2).
The idler light 3 and the terahertz wave 4 generated at this time have a spatial distribution, and the wavelengths of these waves continuously change in accordance with emission angles of the waves. The generation of the idler and terahertz waves in this single path arrangement is referred to as THz-wave parametric generation (TPG).
It is to be noted that a basic optical parametric process is defined by disappearance of one pump photon, and simultaneous generation of one idler photon and one signal photon. In a case where the idler light or signal light resonates, when pump light intensity exceeds a certain threshold value, parametric oscillation occurs. The disappearance of one pump photon and the simultaneous generation of one idler photon and one polariton are the stimulated Raman scattering, and are included in the parametric interaction in a broad sense.
However, there have been problems that the terahertz wave generated in a terahertz wave generation device having the above single path arrangement is very weak and that a large part of the wave is absorbed while the wave travels several hundreds of micrometers through the non-linear optical crystal. For example, owing to the absorption of the LiNbO3 crystal, the terahertz wave indicates a small value of about 0.1% while the wave travels along a length of 3 mm.
To solve the problems, Patent Documents 1, 2 are disclosed. Moreover, Patent Document (non laid-open) 3, Non-Patent Documents 1, 2 and the like are related to the present invention.
FIG. 2 is a schematic diagram of a submillimeter wave generation device disclosed in Patent Document 1. As shown in this drawing, when idler reflection mirrors M1, M2 are constituted in a specific direction (an angle θ) with respect to broad idler light 3 described above, the intensity of the idler light 3 of the specific direction can be increased. It is to be noted that in this drawing, 5 is a laser unit which emits laser light as pump light 2, and 6 is a prism which guides terahertz wave 4 to the outside. The prism 6 is formed of a material having a small absorption coefficient with respect to the terahertz wave.
FIG. 3 is a schematic diagram of a terahertz wave generation device disclosed in Patent Document 2. As shown in this drawing, first laser light 7 having a single frequency is used as pump light 2, and another second laser light 8 having a single frequency is optically injected in a generation direction of idler light 3. In consequence, an output of the generated terahertz wave can largely be increased. In this drawing, reference numeral 9 is a prism array constituted by arranging a plurality of prisms 6 described above.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 09-146131, “Submillimiter Wave Generation Device”
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2002-072269, “Terahertz Wave Generation Method and Device”
[Patent Document 3]
Japanese Patent Application No. 2003-107885, non laid open
[Non-Patent Document 1]
K. Kawase et. al., “Arrayed silicon prism coupler for a THz-wave parametric oscillator”, Applied Optics, vol. 40, No. 9, pp. 1423 to 1426, 2001
[Non-Patent Document 2]
K. Kawase et. al., “Terahertz wave parametric source”, Journal of Physics D; Applied Physics, vol. 35, No. 3, pp. R1 to R14, 2002
As described above, when the pump light 2 (the excitation light) is struck on a non-linear optical crystal 1 having the Raman activity and the far infrared activity in a constant direction, idler light 3 and terahertz wave 4 are generated. The idler light 3 and the terahertz wave 4 generated at this time have a spatial distribution in a direction which satisfies a requirement for phase matching, and wavelengths of these waves continuously change in accordance with an emission angle.
Moreover, when the mirrors M1, M2 are constituted in a specific direction with respect to the idler light 3 as shown in FIG. 2, intensities of the idler light 3 and the terahertz wave 4 in the specific direction can be increased.
However, to take out the terahertz wave 4 generated in the crystal, total reflection conditions need to be avoided on a crystal end surface. Therefore, as shown in FIGS. 2, 3, the prism 6 and the prism array 9 have heretofore been formed of a material having a refractive index smaller than that of the non-linear optical crystal 1, and attached to the non-linear optical crystal 1 to take the terahertz wave 4 out of the crystal.
However, such conventional means have the following problems.
(1) Since a generation point of the terahertz wave is present in the non-linear optical crystal, there is a large absorption amount in the crystal. For example, as described above, owing to the absorption of the LiNbO3 crystal, the terahertz wave decreases to about 0.1% while the wave travels along a length of 3 mm.
(2) Since the terahertz wave is obliquely incident into an interface between the non-linear optical crystal and the prism, transmittance decreases, and output efficiency of the terahertz wave is decreased.
(3) An excessively weak output of the terahertz wave is taken out of the single prism. When the prism array is used, the output increases. However, the terahertz wave 4 is scattered and, due to the prism array including a plurality of prisms, a wave front of the terahertz wave is distorted, an emitted beam does not diametrically have a circular shape (or an elliptic shape), and there has been a trouble in application and development.
That is, if the terahertz wave 4 has a rotationally symmetric or ellipsoidic output distribution around an optical axis of the wave, it is possible to adapt a ray trace calculation for the terahertz waves. However, in the conventional means, the generated terahertz wave largely deviates from rotation symmetry. Therefore, it has been difficult to apply a gaussian optical system.
The present invention has been developed to solve such problems. That is, an object of the present invention is to provide a method and an apparatus for generating the terahertz wave in which absorption in a crystal can largely be reduced, output efficiency from an interface to the outside can be increased, and it is possible to obtain a terahertz wave output distribution close to rotation symmetry to which a gausssian optical system is easily applied.